Microwave waveguide filters, which usually comprise rectangular cross section tubular bodies, have generally been made of the alloy Invar 36. This is because these filters must not change their shapes, significantly, with temperature changes, since changes in shape affect the electrical characteristics of the device. Invar 36 has a very low coefficient of thermal expansion.
As long as weight of the filter is not a factor, Invar 36 is a satisfactory material to use for its construction. However, in applications such as communications satellites, where reduction in weight becomes of great importance, it is highly desirable to make such filters of a material which is not only lighter than Invar 36 but which in addition to the desirable electrical characteristics and the strength needed for such an application, has the low coefficient of thermal expansion of Invar 36.
One answer to this need is to make light weight microwave waveguide filters of graphite fibre epoxy composite material. This material is available in the form of narrow tapes which are composed of carbon fibres and uncured epoxy resins. The tape may be wound on a mandrel of suitable shape, until the tape is several layers thick so that the resultant article is a hollow tube which may have a rectangular cross section. The walls of the tube may be made of any desired thickness by varying the number of layers of tape.
After the layers of tape have been wound on the mandrel, the mandrel, so taped, is baked under ordinary atmospheric pressure at the curing temperature of the epoxy resin. The article so formed is then removed from the mandrel to make a light weight, strong, rigid article. In the past, however, these tubes have had certain disadvantages which greatly detracted from their efficiency as microwave waveguide filters. They are only slightly conductive, and although it has been known that the inner surface of a waveguide filter must have good electrical conductivity, past attempts to form a highly conductive surface on the graphite epoxy composite material have not been entirely successful. Another past disadvantage of the graphite fibre epoxy composite material has been its irregular surface. It is necessary that the inner surface of a microwave waveguide filter be as nearly plane as possible.
It was also found that the outside surface of the shaped graphite fibre epoxy article was subject to severe blistering due to voids which caused outgassing.
The present invention comprises an article made of graphite fibre epoxy composite material and having a good electrically conductive surface composed of a thin flash of nickel and a heavier coating of cooper. When used as a microwave waveguide filter which has a tubular shape, the inner surface of the tubular filter may first be made smooth and glossy and the glossy surface may then be made suitable for electroless plating by converting it to a slightly roughened finish. By using an alloy solder having a melting point of not more than about 150.degree. C, areas of solder may be provided, where needed, on the copper coating.